1. Field of the Invention
The present invention relates to a multi-electron beam source and to an image display device using the same, having a large number of electron-emitting elements arranged in a plurality of rows.
2. Belated Background Art
A cold cathode element disclosed, for example, by M. I. Elinson et al. is known as the element which is capable of emitting electrons with a simple structure (Radio Engineering Electron Physics, Vol. 10, pp. 1290-1296, 1965).
The element is based on the phenomenon that electron emission occurs when an electric current is caused to flow through a film having a small area formed on a substrate in parallel to the film surface thereof, which is generally called a surface conduction type electron-emitting element.
Known surface conduction type electron-emitting elements include: one using an SnO.sub.2 (Sb) thin film developed by Elinson et al. as described above; one based on an Au film (G. Dittmer: "Thin Solid Films" Vol. 9, p. 317 (1972)); one based on an ITO film (M. Hartwell and C. G. Fonstad: IEEE Trans. ED Conf., p. 519 (1975)); one based on carbon film (Hisashi Araki et al.: Shinku, Vol. 26, No. 1, p. 22 (1983)): and one using Pd, in place of the above SnO.sub.2, Au or ITO, as a material of the electron-emitting portion (Japanese Patent Application Laid-Open No. 1-279542).
In addition to the surface conduction type electron-emitting elements, reported are a cold cathode element such as an MIM type electron-emitting element, and a finely fabricated field emission electron gun.
These cold cathode elements have advantages of high electron emission efficiency, a simple structure for easy fabrication, and practicability of arrangement of a large number of elements in array on a single substrate.
The inventors of the present invention already proposed a device, as shown in FIG. 1, in which a large number of such cold cathode elements are densely arranged in an array and the resistance of the electric wiring therefor is reduced. In FIG. 1, ES represents an electron-emitting element, and E.sub.1 to E.sub.m+1 denote respectively a distributing electrode, the electron-emitting elements and the distributing electrodes forming an array having m rows of electron-emitting elements. This functional region is called an electron-emitting element part.
In this device, any one of the rows may be selectively driven. For example, when a driving voltage V.sub.E [V] is applied only to an electrode E.sub.1 and 0[V] is applied to electrodes E.sub.2 to E.sub.m+1, the driving voltage V.sub.E [V] is applied only to the elements in the first row, whereby only the elements in that row are caused to emit electron beams. Generally, in order to drive the n-th row, it suffices to apply V.sub.E [V] to electrodes E.sub.1 to E.sub.n and to apply 0[V] to electrodes E.sub.n+1 to E.sub.m+1, and, in the case where none of the columns is to be driven, it suffices to bring all of E.sub.1 to E.sub.m+1 to the same potential (e.g., 0[V]).
Such a multi-electron beam source capable of row-sequential drive is highly promising for use for a flat panel CRT, since an XY-matrix type of electron beam source may easily be formed by providing grid electrodes perpendicularly to the rows of the elements.
In driving the multi-electron beam source as shown in FIG. 1, a problem is involved that a spike-like voltage arises and is applied undesirably to the rows of elements which should be halting. This problem is explained below by reference to FIG. 2 and FIG. 3.
FIG. 2 shows a typical example of the circuit for driving the multi-electron beam source shown in FIG. 1. In FIG. 2, switching elements such as field-effect transistors (FET) are connected in the manner of a totem pole to the distributing electrodes represented by E.sub.1 to E.sub.m+1, where, by suitably controlling gate signals GP.sub.1 to GP.sub.m+1 and GN.sub.1 to GN.sub.m+1 of the respective FET, 0[V] (ground level) or V.sub.E [V] may be selectively applied to each distributing electrode. This functional region is called a driving circuit part.
FIG. 3 is a graph exemplifying the voltage to be applied to each section for driving the multi-electron beam source shown in FIG. 2. In FIG. 3, the folded line 1 shows the change of the driving state in the case where the rows of the elements are sequentially driven with interposition of halting periods, starting from the first row. Such driving is practiced in use for a multi-electron beam source for a flat panel CRT.
In such driving, rectangular voltage pulses of V.sub.E [V] are applied to the distributing electrodes E.sub.1 to E.sub.4 in lapse of time as indicated by the folded lines 2 to 5 in FIG. 3. For example, the difference in voltage between E.sub.1 (folded line 2) and E.sub.2 (folded line 3) is applied to the first-row elements. Thus the voltage V.sub.E is applied to the first-row elements during the first-row driving period as indicated by the driving state line 1. Thereafter in a similar manner, the difference in voltage between E.sub.2 (folded line 3) and E.sub.3 (folded line 4) is applied to the second-row elements, and the difference in voltage between E.sub.3 (folded line 4) and E.sub.4 (folded line 5) is applied to the third-row elements.
However, according to actual observation with an oscilloscope, as shown by the folded lines 6 and 7 in FIG. 3, a spike-like voltage SP(+) (indicated by the dotted line) or SP(-) (indicated by the solid line) is applied at the instant when another row of the elements is turned on or off.
Such a spike-like voltage applied to the electron-emitting elements tends to cause undesired emission of electron beams in the halting period. If such a device is used for an electron beam source of a flat panel CRT, undesired light emission is caused by the spike-like voltage at the time when light should not be emitted, whereby the image contrast is impaired disadvantageously.
Such spike-like voltage arises presumably because the timing of turn-on or turn-off of respective electrodes deviates from the intended time shown by the aforementioned folded lines 2 to 5. Specifically, in the first element row, the electrodes E.sub.1 and E.sub.2 should be simultaneously switched as 0[V].fwdarw.V.sub.E [V] (or V.sub.E [V].fwdarw.0[V]) at the time where the second or subsequent element row is to be turned on (or off). If the timing of turn-on or turn-off deviates from the ideal timing, a spike-like voltage comes to be applied.
The polarity of the spike-like voltage, a positive voltage spike SP(+) or a negative voltage spike SP(-), depends on which one of E.sub.1 or E.sub.2 the voltage applied earlier to.
The deviation in timing of the voltage application to each electrode results from the following causes: deviation in timing of the gate signals GP.sub.1 to GP.sub.m+1 and GN.sub.1 to GN.sub.m+1 of FET's of the driver circuit as shown in FIG. 3 described above, and variation of time of switching owing to variation in characteristics of each FET.
Complete elimination of the spike-like voltage SP by adjustment of the timing of the gate signals and/or control of the variation in FET characteristics is extremely difficult technically, and is considered not to be practical.
As described above, various problems are involved in the arrangement of a number of cold cathode elements as shown in FIG. 1. Similar problems are involved in arrangements different from that of FIG. 1. For example, multi-electron beam sources shown in FIG. 12 and FIG. 18 also involve problems of occurrence of unintended application of spike-like voltage to the electron-emitting elements.
The multi-electron beam sources shown in FIG. 12 will be explained.
FIG. 12 shows an arrangement of L rows of electron-emitting elements, in which ES denotes an electron-emitting element, and E.sub.p1 to E.sub.pl and E.sub.m1 to E.sub.ml denote wiring electrodes. In this device, each of the L rows of the elements are capable of being driven in arbitrary combination thereof. A desired row of elements can be selectively driven by application of voltage V.sub.E [V] to the electrode among E.sub.p1 to E.sub.pl for the row of elements to be driven and application of 0 [V] to the electrodes for the other rows of elements not to be driven, with application of voltage 0 [V] to all of the electrodes E.sub.m1 to E.sub.ml. Naturally, the elements can be scanned, row by row, sequentially.
Such a multi-electron beam source in combination with grid electrodes orthogonal to the element rows enables construction of an XY matrix type electron beam source, and is promising for use for display apparatuses such as a flat plate type CRT.
The multi-electron beam source shown in FIG. 12, however, when driven by an electric circuit, causes occurrence of application of undesired spike-like voltage to element rows which should be halting. This problem is explained by reference to FIG. 13 and FIG. 14.
FIG. 13 shows a typical example of the electric circuits for driving the multi-electron beam source of FIG. 12. In FIG. 13, switching elements such as field effect transistor (FET) are connected in a manner of a totem pole to the distributing electrodes represented by E.sub.p1 to E.sub.pl. By suitably controlling gate signals GP1 to GPl and GN1 to GNl, for the respective rows of FET, 0 [V] (ground level) or V.sub.E [V] may be selectively applied to each wiring electrode. To the respective electrodes E.sub.m1 to E.sub.ml, voltage 0 [V] (ground level) is applied.
FIG. 14 exemplifies the voltages to be applied to each part for driving the multi-electron beam source with the electric circuit shown in FIG. 13. In FIG. 14, a case is considered in which the element rows are sequentially driven from the first row with interposition of halting periods as shown by FIG. 14 1. (A multi-electron beam source for a flat plate type CRT, etc. is driven generally in such a driving method.)
In such a driving method, rectangular voltage pulses of V.sub.E [V] are applied to the wiring electrode E.sub.p1 to E.sub.p3 at timings shown in 2 to 4 of FIG. 14, while voltage 0 [V] is applied to the wiring electrodes E.sub.m1 to E.sub.ml as shown in 5 of FIG. 14. For example, the difference in the voltage between 2 and 5 in FIG. 14 is applied to the first-row electron-emitting elements, whereby V.sub.E [V] is applied thereto during the time only of driving of the first row element as shown in 1 of FIG. 14. In a similar manner, the difference in voltage of between 3 and 5 in FIG. 14 is applied to the second-row electron-emitting elements, and the difference in voltage between 4 and 5 in FIG. 14 is applied to the third-row electron-emitting elements.
However, according to actual observation with an oscilloscope, as shown by the folded lines 6 to 8 in FIG. 14, a spike-like voltage SP was found to be applied at the instant when another row of the elements is turned on or off.
The occurrence of the spike-like voltage SP is considered to result from instantaneous malfunction of FET caused by electric noise, electrical induction by mutual induction between adjacent wiring electrodes, deformation of applied voltage wave form by inductance, capacitance, resistance, etc. of the wiring electrodes before the voltage reaches the electron-emitting elements, and so forth.
If the amplitude of the spike-like voltage is relatively large, an electron beam is emitted from the electron-emitting element at an undesired point of time. This causes unwanted light emission which is irrelevant to the image to be displayed on a flat plate type CRT display, giving noise of the image or low contrast of the image, disadvantageously.
The description above explains the problems involved in the multi-electron beam source shown in FIG. 12. The problems involved in the multi-electron beam source shown in FIG. 18 are explained below.
In FIG. 18, ES denotes an electron-emitting element, E.sub.c1 to E.sub.CM denote wiring electrodes in the column direction, and E.sub.R1 to E.sub.RN denote wiring electrodes in the row direction. In this multi-electron beam source, electron-emitting elements of M.times.N in number are arranged in a matrix, and the elements are connected electrically by the column-direction wiring electrodes and the row-direction wiring electrodes to form a wiring matrix. The element groups arranged in parallel to the X direction are called element columns, and the element groups arranged in parallel to the Y direction are called element rows. Thus the element matrix is constructed from M element columns and N element rows.
Such a multi-electron beam source is generally driven, column by column, sequentially and selectively. Being different from the cases shown in FIG. 1 and FIG. 12, the ones of FIG. 18 are capable of emitting electron beams from desired electron-emitting elements selectively in the selected element columns. This is explained by reference to FIG. 19 to FIG. 22.
FIG. 19 is a graph showing a general characteristic of a cold cathode element used as an electron-emitting element ES, in which the abscissa shows the voltage applied to the element and the ordinate shows intensity of the electron beam emitted from the element. Generally, no electron beam is emitted from the element at an applied voltage lower than a certain threshold voltage V.sub.th, and at the voltage exceeding the threshold voltage V.sub.th, the intensity of the emitted electron beam increases with the increase of the applied voltage. Accordingly, a voltage V.sub.E can readily be set such that no electron beam is emitted at the voltage V.sub.E /2 and an electron beam is emitted at the voltage V.sub.E. A driving method utilizing such voltage V.sub.E is described below.
As an example, a case is considered in which the first element column is selected from the multi-electron beam source, and electron beams are allowed to be emitted from the second to fifth rows of the selected column. FIG. 20 shows the voltage application to the respective wiring electrodes for this purpose. Among the column-direction wiring electrodes E.sub.C1 to E.sub.C6, the voltage 0 [V] is applied to the first column wiring electrode E.sub.C1, and the voltage V.sub.E /2 [V] is applied to other electrodes E.sub.C2 to E.sub.C6. Among the row-direction wiring electrodes E.sub.R1 to E.sub.R6, the voltage V.sub.E [V] is applied to the second to fifth row electrodes E.sub.R2 to E.sub.R5, and the voltage V.sub.E /2 is applied to E.sub.R1 and E.sub.R6. The voltage applied to each of the respective electron-emitting elements is the difference in voltage between the column-direction wiring electrode and the row-direction wiring electrode connected thereto. Therefore, V.sub.E [V] is applied to the solid-marked electron-emitting elements; V.sub.E /2 [V] is applied to the obliquely striped or laterally striped electron-emitting elements; and 0 [V] is applied to the dot-marked electron-emitting elements in FIG. 20. Therefore, the voltage higher than the threshold for electron emission is applied to the intended electron-emitting elements to emit electron beams, whereas no electron beam is emitted from other electron-emitting elements.
As described above by reference to examples, the element columns can be selected by applying 0 [V] to the column-direction wiring electrode of the column of the element to be driven and applying V.sub.E /2 [V] to other column-direction wiring electrode. Further, the intention can be achieved by applying V.sub.E [V] to the row-direction wiring electrode for the row to allow electron beam emission and applying V.sub.E /2 [V] to the wiring electrodes for the rows to allow no electron beam emission. In the above-described driving method, the voltage applied to the row-direction wiring electrode to electron beam emission is fixed to V.sub.E [V], thereby intensity of the emitted electron beam is also fixed to a definite value I.sub.1. The intensity of the emitted electron beam can be controlled in the range of from 0 to I.sub.1 by selecting the applied voltage in the range of from V.sub.th [V] to V.sub.E [V] in accordance with the electron-emitting characteristic of the element as shown in FIG. 19.
Such a multi-electron beam source constitutes by itself an XY matrix type electron beam source, which is promising for the uses of display apparatus such as a flat plate type CRT.
However, in practical driving of a multi-electron beam source of FIG. 18 with an electric circuit, spike-like voltage is found to be caused and applied to the electron-emitting element. FIG. 21 to FIG. 23 are drawings for explaining such problems.
FIG. 21 shows a typical example of the electric circuits for driving the multi-electron beam source of FIG. 18. In FIG. 21, switching elements such as field effect transistor (FET) are connected in a manner of a totem pole to the wiring electrodes. The circuit connected to the column-direction wiring electrodes E.sub.C1 to E.sub.CM applies 0 [V] or V.sub.E /2 [V] selectively thereto, and the circuit connected to the row-direction wiring electrodes E.sub.R1 to E.sub.RN applies V.sub.E [V] or V.sub.E /2 [V] selectively thereto. The desired voltage can be selectively applied to the respective wiring electrodes by suitably controlling gate signals GP.sub.C1 to GP.sub.CM, GN.sub.C1 to GN.sub.CM, GP.sub.R1 to GP.sub.RN, and GN.sub.R1 to GN.sub.RN.
FIG. 22 is a drawing for explaining an example of an arbitrary driving pattern of the multi-electron beam source. The driving pattern is explained for the case where electron beams are emitted from the multi-electron beam source in a pattern of the letter "E" as shown by shadowing in FIG. 22. Generally a multi-electron beam source is driven such that element columns are driven sequentially, column by column, in the order of first column, the second column, the third column, and so forth. In such a manner, the "E" type pattern of FIG. 22 is completed. In FIG. 23, 1 shows the change of driving steps with time.
For driving the element columns, the voltage is applied to the respective wiring electrodes as described above. For example, the first column elements are driven by application of driving voltage to the wiring electrodes in the same manner as described in the explanation of the driving procedure for FIG. 21. In FIG. 23, 2 to 9 show the change with time of the voltages applied to wiring electrodes E.sub.C1 to E.sub.C4, and E.sub.R1 to E.sub.R4.
In driving of the electron beam source with the electric circuit shown in FIG. 21 according to the above procedure, occurrence of unwanted spike-like voltage was observed in the voltage applied practically to respective electron-emitting elements by an oscilloscope. For example, in the three elements denoted by A, B, and C in FIG. 21, the observed waveforms of the applied voltage were as shown by 10 to 12 in FIG. 23. In FIG. 23, SP(n) and SP(T) denote the unintended spike-like voltages.
The occurrence of the spike-like voltage SP(n) is considered to result from instantaneous malfunction of FET caused by electric noise, electrical induction by mutual inductance between adjacent wiring electrodes, deformation of applied voltage waveform by inductance, capacitance, resistance, etc. of the wiring electrodes before the voltage reaches the electron-emitting elements, and so forth. The main cause of occurrence of SP(T) is considered to be due to a time lag of the operation of FET for driving the column-direction wiring electrodes and the operation of the EFT for driving the row-direction wiring electrodes.
If the amplitude of the spike-like voltage is relatively high, an unwanted electron beam is emitted from the electron-emitting element at unintended time even though the emission occurs for a limited short time. This causes unwanted light emission which does not correspond to the image to be displayed on a flat plate type CRT display, giving noise of the image or low contrast of the image, disadvantageously.